X-ray generator



Z. J. ATLEE X-RAY GENERATOR Jan. 23, 1962 3 Sheets-Sheet 1 Filed Oct.27, 1958 INVENTOR. ZED J. ATLEE ATTORNEYS Jan. 23, 1962 2. J. ATLEE3,018,398

X-RAY GENERATOR Filed Oct. 27, 1958 3 Sheets-Sheet 2 FIGJI INVENTOR. ZED J, ATLEE A T TORNELS BY BUCKHORN,CHEATHAM &BLORE Jan. 23, 1962 2. J.ATLEE 3,018,398

X-RAY GENERATOR Filed Oct. 27, 1958 3 Sheets-Sheet 3 INVENTOR. ZED 3'.AT LEE BY BUCKHORN,CHEATHAM ABLORE ATTORNEYJ ilnited States Patent3,018,398 X-RAY GENERATOR Zed J. Atlee, Chicago, 111., assignor toDunlee Corporation, Chicago, IlL, a corporation of Illinois Filed Oct.27, 1958, Ser. No. 769,651 11 Claims. (Cl. 31359) The present inventionrelates to X-ray generators and more particularly to construction ofX-ray generators permitting better definition in the X-ray picturestaken therewith.

To overcome some of the problems attendant with the heating of the anodetarget in the operation of an X-ray generator, rotating anodes have beenused. While such anodes have helped solve the problem of overheating,they have raised an additional problem in trying to secure X-raypictures of good definition. In the operation of an X-ray generator,some of the primary electrons generated at the cathode are diverted fromthe desired path and will strike the anode at portions other than withinthe desired focal area of the electron beam upon the anode to generateundesirably large quantities of spurious X-rays if the material hit isof high atomic number. Also, the heated portions of the generator willgive rise to secondary electron emission which will also strike variousportions of the anode target giving rise to generation of spuriousX-rays. Obviously, to secure the best possible shadow picture of anobject being X-rayed, it is desirable to have the source of X-rays assmall as possible, a pinpoint source theoretically being the mostdesirable, but, being, of course, impractical to obtain in an X-raygenerator under the present state of knowledge. However, it is possibleto form a reasonably small focal area on an X-ray generator targetmember so that a picture of superior definition could be obtained if theX-rays striking the X-ray sensitive medium were restricted to those fromsuch focal area. Heretofore, however, the spurious X-rays generated bythe diverted primary electrons and the secondary electrons as mentionedabove have been essentially unrestricted and have been permitted to passfreely from the tube with the result that the definition of the picturebeing taken has been lessened by reason of the various spurious X-raysstriking the X-ray sensitive medium.

It is, therefore, an object of the present invention to provide an X-raygenerator in which the X-rays leaving the generator in the direction ofan object being X-rayed are limited essentially to those arising fromthe desired focal area on the anode target.

It is a more specific object of the invention to shield an object beingX-rayed from X-rays generated at the target of an X-ray generator otherthan from those Xrays generated at the desired focal area of the target.

Another specific object of the invention is to provide an X-raygenerator with means to shield the anode target member from the divertedprimary and secondary electrons so as to prevent origination of spuriousX-rays outside of the target area.

Other objects and advantages of the invention will be come more apparenthereinafter.

In accordance with an illustrated embodiment of the present invention, arotating anode type X-ray generator is provided with a diaphragmpositioned about the path of the useful beam of X-rays desired to beemitted from the generator. The diaphragm is comprised of a material ofhigh atomic number so that it is substantially opaque to X-rays and isprovided with an aperture through which the useful beam of X-rays maypass. Portions of the diaphragm exposed to a view of the object beingX-rayed are preferably provided with a lamina or coating of material oflow atomic number, such as beryllium, so that such surfaces are shieldedfrom secondary electrons thus to minimize rise of spurious X-rays fromsuch surfaces. Portions of the anode target member other than thatforming the focal area for the electrons preferably are also providedwith a coating of low atomic number again to prevent such portions frombeing a source of spurious X-rays.

For a more detailed description of the invention, reference is made tothe following specifications and accompanying drawings wherein:

FIG. 1 is a side elevation partially in section of an X-ray generatormade in accordance with the invention;

FIG. 2 is a bottom view of the anode portion of the generator;

FIG. 3 is an end view of the anode target member looking in thedirection of the arrows 3-3 of FIG. 2;

FIG. 4 is an enlarged fragmentary sectional view taken substantiallyalong line 44 of FIG. 2;

FIG. 5' is a view of the diaphragm member taken along line 55 of FIG. 4;

FIG. 6 is a fragmentary side elevation of a modified embodiment of theinvention shown partly in section;

FIG. 7 is a fragmentary bottom view of the anode portion of thegenerator looking in the direction of the arrows 77 of FIG. 6;

FIG. 8 is an end view of the target member of the embodiment of FIG. 6looking in the direction of the arrows 8-8 of FIG. 6;

FIG. 9 is a fragmentary side elevation partially in section of stillanother embodiment of the invention;

FIG. 10 is a fragmentary elevation of the anode portion of the generatorshown in FIG. 9;

' FIG. 11 is an end view of the anode taken in the direc tion of thearrows 11-11 of FIG. 9;

FIG. 12 is a side elevation partially in section of a further embodimentof the invention;

FIG. 13 is a sectional view taken substantially along line 13-13 of FIG.12; and

FIG. 14 is a side elevation partially in section of still anotherembodiment of the invention.

Referring first to FIGS. 1 to 5, inclusive, the X-ray generator showntherein comprises an envelope 10 of glass or other suitable materialhaving a cathode 12 mounted at one end thereof and an anode 14 mountedin the other end thereof. The anode 14 is of the rotary type andcomprises a rotor body 16 suitably supported by bearing structure (notshown) upon a stemor spindle 18 which extends outwardly of the envelopethrough a conventional glass-to-metal seal structure 20. Extending fromthe rotor 16 towards the cathode 12 is a shaft 26 wich supports a targetmember 28 secured to the shaft by a. cap nut 30. The target disc 28 ispreferably of tungsten or other metal of high atomic number and the nut30 and shaft 26 are preferably of molybdenum or other suitable metalhaving high strength at high temperatures. As will be noted, the target28 is formed with a peripheral frusto-conical target surface portion 32and a flat center portion 34 which extends at right angles to the shaft26.

The cathode 12 is provided with a head portion 40 having a focussing cuprecess 42 provided therein in which is mounted a filament 44 adapted tobe energized through leads 46 from a suitable source of electricalenergy. The filament 44 and cup 42 are designed to generate and focus abeam of electrons indicated at 48, upon the target portion 32 of theanode target 28. The beam 48 is preferably of rectangular cross-sectionWith its longer axis extending vertically as the generator is shown inFIG. 1. The beam 48 is focussed to impinge upon the target area 32 ofthe rotor 28 over a focal area 50. Preferably, the angle of inclinationof the target portion 32 is such that as viewed at right angles to thebeam 48, the focal spot 50 is square. This is, of course, utilization ofthe conventional line-focus principle.

As will be recognized by those skilled in the art, the structuredescribed heretofore is that of a more or less conventional rotatinganode X-ray generator or tube. In such tubes, secondary electrons aregenerated from many sources and strike portions of the anode surfaceother than the focal area 50. These secondary electrons give rise toX-rays, some of which will be emitted toward the object being X-rayed.Also, electrons are deflected from the primary beam 48 and strike theanode outside of the target area 50. It will be apparent that in takingan X-ray picture, the definition of the picture obtained will bedependent upon the smallness of the focal area from which X-rays areemitted. The spurious X-rays generated outside of the focal area in aconventional rotating anode tube, such as from the surface portion 34and nut 30 of the target and also from the portions of the targetimmediately adjacent the focal spot 50, in effect cause an enlargementof the focal area so that the picture obtained loses some of itsdefinition. The X-ray generator of the invention is provided with meansto shield the object being X-rayed and the X-ray film from X-rays otherthan those generated at the focal area so that pictures of improveddefinition may be obtained.

In accordance with the invention, the surface portion 34 of the target32 is preferably protected from electron bombardment by a coating 51 ofa high temperature material of low atomic number, such as, for example,graphite. The nut 30 is likewise provided with a coating 52 of such lowatomic number material. Low atomic number materials, it will berecognized, are poor sources of X-rays so that relatively few X-rayswill be generated by electrons which may strike such materials.Furthermore, the coatings serve to stop any electrons impinging thereonto prevent the electrons from reaching the heavy metal beneath thecoating. Thus, such coatings substantially eliminate the coated surfacesas sources of X-rays which might otherwise be emitted therefrom.

The generator is also provided wiht a diaphragm means capable ofscreening off X-rays generated by secondary and diverted electrons onportions of the target surface 32 adjacent the focal area 50. In thepresently described embodiment of the invention, the diaphragm meansincludes a support arm 60 suitably secured to the stem 18 adjacent theglass-to-metal seal 20. The arm 60 extends along the rotor body 16 to apoint just rearwardly of the target disc 28 at which point it is turneddownwardly as the tube is shown in FIG. 1. To the end of the arm 60 issecured a diaphragm disc 62 which consists of a sheet of material ofhigh atomic number, such as tungsten, which is substantially opaque toX-rays. The disc 62 is illustrated as secured to the end of the arm 60by means of screws 64, though other suitable means can be used. Toprovide an aperture through the disc 62 for the X-rays generated at thefocal spot 50, an opening 66 is provided in the disc and which openingis preferably of a size such as to provide a sight of substantially onlythe focal spot 50 from any point within a predetermined useful field oftheX-rays generated at the focal spot 50. It will be understood thatthis useful field is defined by an imaginary cone of X-rays, indicatedat 68, diverging from the target focal spot 50, as shown in FIG. 4.Preferably, the opening 66 is frusto-conical and defines the outline ofthe useful field 68. Preferably, a layer or coating of a material of lowatomic number is positioned over the face of the disc 62 facing awayfrom the anode target 28 so that secondary electrons cannot reach suchsurface of the disc and generate interfering X-rays therefrom. In theillus trated embodiment of the invention, the disc 62 is provided with acover or jacket 70 of a material of low atomic number, such as, forexample, beryllium, and which jacket includes a bottom portion 72 whichengages against the bottom of the disc 62 and a side wall portion 74which surrounds the peripheral edge of the disc so as to prevent .anyaccess of electrons to the bottom surface of the disc. The jacket may besecured to the disc in any suitable manner. In the illustratedembodiment, the peripheral edge 74 is provided with an upstanding flange76 extending about two-thirds of the distance about the periphery andwhich flange defines a groove 78 for receiving a snap ring 80 whichextends over the upper edge of the disc 62 to hold the jacket inposition.

Preferably, the diameter of the disc 62 is such as to mask the anodesurface from points within the useful field of the generator for asubstantial distance around the focal spot 50. Thus, X-rays from theareas surrounding the focal spot 50 will be cut off with the result thatX-ray shadow pictures of superior definition can be obtained.

FIGS. 6, 7 and 8 illustrate a modified embodiment of the inventioncomprising an envelope 80 in which is mounted in any suitable manner acathode 82 and a rotary anode 84. The cathode 82 may be of aconstruction similar to the cathode 12 described above. The anode 34comprises a suitably supported rotor 86 from which extends a shaft 88upon which is mounted a target member 90 of tungsten or like metal .ofhigh atomic number capable of producing X-rays when struck withelectrons. The target member 90 is formed with a peripheralfrustoconical target portion 92 and a flat center portion 94 whichextends at right angles to the shaft 88. The target member 90 issuitably secured to the shaft 88 as by means of a nut 96. Mounted on thetarget member and covering the center portion 94 and the nut 96 thereofis a hat-like covering member 98 formed of graphite or other suitablehigh temperature material of low atomic number. The hat 98 is suitablysecured in position as by a pin 100 extending through the hat into anaperture provided therefor in the nut 96.

The cathode 82 may be constructed in substantially the same manner asthe cathode 12 described above and is provided with means to generateand focus a beam of electrons 104 upon the target portion 92 of theanode target member 90. The beam 104 is preferably of rectangular crosssection with its longer axis extending vertically of the generator asshown in FIG. 6 so as to form a rectangular focal area 106 on the targetmember as indicated in FIG. 8.

Diaphragm means are provided in the generator including a support arm110 suitably secured to a stationary portion of the generator and uponwhich is mounted a diaphragm element 112 which is curved arcuatelyconcentrically of the axis of rotation of the target member 90. Thediaphragm element 112 comprises a sheet-like core 114 of X-ray opaquematerial such as, for example, tungsten, encapsulated in a material oflow atomic number, such as beryllium, graphite, boron or other poor X-ray generating material, indicated at 116. The core 114 is provided withan aperture 118 to permit the transmission of a useful field of X-raysfrom the focal area 106 of the anode, such field being indicated by thedotted lines 120. While the aperture 118 may be circular in outline, asshown in the previously described embodiment and which shape facilitatesmanufacture, preferably, the aperture is square in outline as indicatedin FIG. 7 so a to conform to the projected shape of the focal spot 106.

As will be apparent, the hat 98 will shield the center portion 94 andthe nut 96 of the anode from electrons so as substantially to eliminatesuch portions as sources of spurious X-rays. The diaphragm element 112will in turn screen off portions of the target member adjacent the focalarea 106 from the sight of. the useful field, thus to secure betterdefinition in the resulting picture as described hereinabove. The curvedconfiguration of the diaphragm 112 is of advantage in maintaining a moreuniform electrical field within the tube so as to cause less deflectionof electrons from the electron beam 104 and substantially to eliminatearcing between the diaphragm and the envelope 80. The curved diaphragmalso in creases the area of X-ray protection which is extremelydesirable from a safety standpoint.

Reference is now made to FIGS. 9, l0 and 11 which illustrate a stillfurther embodiment of the invention. In these views, there is shown anX-ray tube comprising an envelope 130 having a cathode 132 suitablysupported at one end and a rotary anode 134 in the opposite end thereof.The cathode 132 is provided with a focussing cup and electron generatingfilament of the type described above for generating and focussing a beamof electrons 136 upon the target member of the anode 134. The anode 134includes a rotor body 138 from which extends a shaft 140 for supportinga target member 142 of tungsten or like material capable of generatingX-rays efiiciently when bombarded with electrons. The target member 142is formed with a flat center portion 144 which is normal to the axis ofrotation of the target member and with a surrounding frusto'conicaltarget portion 146 upon which the electron beam 136 is focussed. Thetarget portion 146 extends obliquely with respect to the axis of thebeam 136 so as to utilize the line-focus principle. The target member142 is secured to the shaft 140 by means of a cap nut 148. The surfaceof the center portion 144 and the nut 148 preferably are provided with acoating of a high temperature material of relatively low atomic numberhaving relatively poor X-ray generating efiiciency, an example of suchmaterial being graphite. Other suitable materials will be apparent tothose skilled in the art. Disposed about the target member 142 is a hoodmeans 154 comprising a cup-shaped body 156 of material, such as, forexample, tungsten, relatively opaque to X-rays. The body 156 is mountedcoaxially of the target member 142 with the closed end of the bodyfacing the cathode 132. The body 156 is supported on an arm 169 suitablysecured to a stationary portion of the tube structure, such as, forexample, in the manner of the support of the arm 60 in the firstdescribed embodiment. The body 156 is provided with an opening 164through which the electron beam 136 may pass and is also provided withan aperture 166 for the passage of a useful beam of X-rays 168 generatedat the focal spot 170 of the electron beam 136 on the target portion146. Preferably, the aperture 166 is of a size not substantially greaterthan that necessary to provide a sight of the focal spot 170 from anypoint within the desired useful field of the tube. This field is, ofcourse, as mentioned above defined by an imaginary cone diverging fromthe focal spot 170 in the direction at right angles to the electron beam136.

To prevent the generation of X-rays from the exterior surface of thehood 156 from those portions visible from the useful field of the tube,such surface portions Preferably are provided with a coating of amaterial of low atomic number, such as beryllium, graphite or boron.Preferably, as in the illustrated embodiment the entire hood isencapsulated in such low atomic number material, the encapsulatingcoating being indicated in FIG. 8 at 180.

The hood 154 will, of course, afford more complete X-ray protection forpersonnel. In addition, the hood will more completely shield the X-rayfield of the tube from all portions of the target member except thatviewed through the aperture 166 with the result that better definitionwill be secured. Further, better voltage gradients are formed in thetube and which is of particular concern in higher voltage ranges.

Illustrated in FIGS. 12 and 13 is a tube 210 comprising an evacuatedenvelope 212 of glass or other suitable material within which is mounteda cathode 214 and a rotary anode 216 comprising a target member 218constructed substantially as those hereinbefore described. The flatcenter portion of the target member and the securing nut 220 preferablyare provided with a coating 222 of graphite or other suitable hightemperature material of low atomic number.

Mounted in the path of the useful X-ray beam from the target member 218is a diaphragm element 224 defining a Window in the envelope 212. Asbest shown in envelope 232 of an X-ray transparent material of lowatomic number, such as, for example, beryllium. The disc 230 is providedwith an aperture 234 to permit the transmission of a useful field ofX-rays from the focal area of the target member 218. The diaphragmelement 224 is secured to the mounting ring 228 by suitable means suchas brazing so as to provide a vacuum tight seal. As will be observed,the aperture 234 is positioned in off center relation with respect tothe disc 230 and the diaphragm element 224 is mounted in off centerrelation with respect to the ring 228 with respect to which the aperture234 is centered, so that the diaphragm element extends rearwardly of thetarget member 218 a substantial distance. This construction is preferredto provide a maximum amount of X-ray protection with a disc of givendiameter While maintaining the disc in spaced relation with respect tothe cathode 214 and which is desired, of course, by the voltagedifferences between the cathode and anode.

As will be apparent, the coating 222 will minimize the generation ofX-rays from the central portion of the target member while the diaphragmelement 224 will screen off substantially all of the X-rays generated atportions of the target surface other than at the focal area.

Still another arrangement for supporting a diaphragm element inaccordance with the invention is shown in FIG. 14. The tube thereinillustrated comprises an envelope 250 of glass or other suitablematerial within which is suitably mounted a cathode 252 and rotary anode254 having a target member 256 which may be provided with a protectivecoating 253 of low atomic number over its central portion as previouslydescribed to minimize X-ray generation from such portion. Mounted in thepath of the useful X-ray beam is a diaphragm element 260 supported uponan arm 262 secured to and supported by the envelope through means of aglass-to-metal seal 264 fixed to a neck 266 in the envelope Wall. Thediaphragm element includes a disc 276 of X-ray opaque material such astungsten which preferably is encapsulated within an outer layer 272 of apoor X-ray generating material of low atomic number such as, forexample, beryllium, graphite or boron. The disc 274} is provided with anaperture 274 to permit the transmission through the diaphragm of auseful beam of X-rays. As will be apparent, the cap 258 will minimizethe number of X-rays generated at the center portion of the target andthe diaphragm element 260 will screen off substantially all of theX-rays generated from the target surface at portions other than thefocal area.

Having illustrated and described the preferred embodiments of theinvention, it should be apparent to those skilled in the art that theinvention permits of modifications in arrangement and detail. I claim asmy invention all such modifications as come Within the true spirit andscope of the appended claims.

I claim:

1. An X-ray tube comprising an envelope, a cathode and a rotary anodemounted in opposite ends of said envelope, diaphragm means spaced fromsaid anode and disposed in the path of the useful X-ray beam from saidanode, said diaphragm means comprising a body of a materialsubstantially opaque to X-rays having an aperture therethrough for thepassage of said X-ray beam, said aperture being of a size such as toprovide a sight of substantially only the focal spot of said anode fromany point from within a predetermined useful field of the X-raysgenerated at said focal spot, said body extending radially of suchopening by a distance such as to obscure substantially all of theremainder of said target portion from any point within said usefulfield, means supporting said body with said aperture concentric of saidX-ray beam, and means for shielding the surface of said body facing awayfrom said anode from bombardment by secondary electrons comprising alamina of a material of low atomic number disposed adjacent to andcovering said body surface.

2. In an X-ray tube comprising an envelope, a cathode and an anodemounted in opposite ends of said envelope, diaphragm means disposed inthe path of the useful X-ray beam from said anode, said diaphragm meanscomprising a fiat disc of a material substantially opaque to X-rayshaving an aperture therethrough for the passage of said X-ray beam,means supporting said disc with the same disposed substantially at rightangles to the axis of said X-ray beam and with said aperture concentricof said X-ray beam, and means for shielding the surface of said discfacing away from said anode and the peripheral edge thereof frombombardment by secondary electrons comprising a cup-shaped jacket of amaterial of low atomic number, said jacket including a bottom portionengaging said disc surface and a side Wall portion surrounding said discperipheral edge, means securing said disc and jacket together.

3. An X-ray tube comprising a rotary anode and a cathode, said cathodehaving means for generating and focussing an electron beam ofrectangular cross section upon said anode, said anode comprising atarget portion extending obliquely with respect to said electron beamaxis at such an angle that a substantially square projection of thetarget focal spot is obtained at right angles to the direction of saidbeam, and diaphragm means disposed adjacent but spaced from said targetmember and defining an aperture square in outline symmetrically disposedabout said projection, said diaphragm means comprising a body ofmaterial relatively opaque to X-rays, said aperture being of a size suchas to provide a sight of substantially only said focal spot from anypoint within a predetermined useful field of the X-rays generated atsaid focal spot, said body extending radially of such opening by adistance such as to obscure substantially all of the remainder of saidtarget portion from any point within said useful field, said diaphragmmeans comprising a lamina of a material of low atomic number on thesurface of said body facing an object being X-rayed.

4. In an X-ray tube, a cathode for generating a beam of electrons, ananode comprising a target member supported for rotation about an axisparallel to but displaced from the longitudinal axis of said electronbeam, said target member including a center portion presenting a surfacenormal to said axis of rotation and a surrounding frustoconical targetportion extending into the path of said electron beam, said targetportion extending obliquely with respect to said electron beam axis,said target member comprising a body of metal of relatively high atomicnumber having relatively high X-ray generating efliciency, said centerportion having a coating thereon of a material of relatively low atomicnumber having relatively poor X-ray generating efiiciency, and diaphragmmeans disposed closely adjacent said target member, said diaphragm meanscomprising a body of material relatively opaque to X-rays and providedwith an aperture for the passage of a useful beam of X-rays from thefocal spot on said target portion, said aperture being of a size notsubstantially greater than that necessary to provide a sight of saidfocal spot from the normal useful field of said tube, said diaphragmmeans having a lamina of a material of low atomic number on the surfaceof said body facing an object being X-rayed.

5. In an X-ray tube, a cathode for generating a beam of electrons, ananode comprising a target member supported for rotation about an axisparallel to but displaced from the longitudinal axis of said electronbeam, and hood means disposed about said target member, said hood meanscomprising a cup-shaped body of material relatively opaque to X-raysmounted coaxially of said target member with the closed end of said bodyfacing said cathode, said closed end being provided with an opening forsaid electron beam, the side wall of said cup being provided with anaperture for the passage of a useful beam of X- rays, said aperturebeing of a size not substantially greater than that necessary to providea sight of the focal spot of said electron beam on said target portionfrom the predetermined useful field of said tube, said hood body havinga lamina of a material of low atomic number on the surface portionsthereof in sight from said useful field.

6. In an X-ray tube, a cathode for generating a beam of electrons, ananode comprising a target member supported for rotation about an axisparallel to but displaced from the longitudinal axis of said electronbeam, said target member including a center portion presenting a surfacenormal to said axis of rotation and a surrounding frusto-conical targetportion extending into the path of said electron beam, said targetportion extending obliquely with respect to said electron beam axis,said target member comprising a body of metal of relatively high atomicnumber having relatively high X-ray generating efiiciency, said centerportion having a coating thereon of a material of relatively low atomicnumber having relatively poor X-ray generating efliciency, and hoodmeans disposed about said target member, said hood means comprising acup-shaped body of material relatively opaque to X-rays mountedcoaxially of said target member with the closed end of said body facingsaid cathode, said closed end being provided with an opening for saidelectron beam, the side wall of said cup being provided with an aperturefor the passage of a useful beam of X-rays, said aperture being of asize not substantially greater than that necessary to provide a sight ofthe focal spot of said electron beam on said target portion from thepredetermined useful field of said tube, said hood body having a laminaof a material of low atomic number on the surface portions thereof insight from said useful field.

7. In an X-ray tube, a cathode for generating a beam of electrons, ananode comprising a target member supported for rotation about an axisparallel to but displaced from the longitudinal axis of said electronbeam, said target member including a center portion presenting a surfacenormal to said axis of rotation and a surrounding frustoconical targetportion extending into the path of said electron beam, said targetportion extending obliquely with respect to said electron beam axis,said target member comprising a body of metal of relatively high atomicnumber having relatively high X-ray generating efiiciency, said centerportion having a coating thereon of a material of relatively low atomicnumber having relatively poor X-ray generating efliciency, and hoodmeans disposed about said target member, said hood means comprising acup-shaped body of material relatively opaque to X-rays mountedcoaxially of said target member with the closed end of said body facingsaid cathode, said closed end being provided with an opening for saidelectron beam, the side wall of said cup being provided with an aperturefor the passage of a useful beam of X-rays, said aperture being of asize not substantially greater than that necessary to provide a sight ofthe focal spot of said electron beam on said target portion from thepredetermined useful field of said tube, said hood body beingencapsulated in a material of low atomic number.

8. An X-ray tube comprising a rotary anode and a cathode, said cathodehaving means for generating and focussing an electron beam ofrectangular cross section upon said anode, said anode comprising atarget portion extending obliquely with respect to said electron beamaxis at such an angle that a substantially square projection of thetarget focal spot is obtained at right angles to the direction of saidbeam, and diaphragm means disposed adjacent said target member anddefining an aperture symmetrically disposed about said projection, saiddiaphragm means comprising a body of material relatively opaque toX-rays, said aperture being of a size such as to provide a sight on saidanode of substantially only said focal spot from any point within theuseful field of the X-rays generated at said focal spot, said diaphragmbody being disposed concentrically with respect to the axis of rotationof said anode, the surface portions of said body within sight of saiduseful field being provided with a lamina of a material of low atomicnumber so as to shield such surface portions from impinging electrons.

9. An X-ray tube as set forth in claim 1 wherein said rotary anodeincludes a supporting spindle fixedly sealed to said envelope, and saidmeans supporting said diaphragm means comprises an arm secured at oneend to said spindle and at its other end to said diaphragm means.

10. An X-ray generator comprising a sealed envelope, a rotary anode anda cathode within said envelope, said envelope comprising a tubular glassportion surrounding said anode, said portion having an opening thereinsurrounding the path of the useful beam of X-rays from said anode, ametal mounting ring sealed to said envelope about said opening andextending from the envelope toward said anode, and diaphragm meanssealed to the end of said ring adjacent said anode, said diaphragm meanscomprising a disc of a material of high atomic number and relativelyopaque to X-rays, said disc being encapsulated in a vacuum tightenvelope of 10W atomic number and relatively transparent to X-rays, saiddisc having an aperture therethrough for the passage of a useful beam ofX-rays.

11. An X-ray generator as set forth in claim 1 wherein said envelopecomprises a tubular glass portion having an opening therein adjacent thepath of the useful beam of X-rays from said anode, a metal arm extendingthrough said opening, a metal mounting ring sealed to said envelopeabout said opening and to said arm and supporting said arm, anddiaphragm means secured to said arm and supported thereby in the path ofthe said useful beam of X-rays.

References Cited in the file of this patent UNITED STATES PATENTS1,899,568 Hofmann Feb. 28, 1933 1,953,813 Matsushima Apr. 3, 19342,107,597 Bouwers Feb. 8, 1938 2,496,112 Van Der Tuuk Jan. 31, 19502,617,046 Douma et al Nov. 4, 1952 2,665,391 Bleeksma Jan. 5, 19542,836,749 Atlee May 27, 1958

